Optical interconnects and integrated circuits may be used, in one application, to optically connect one or more optical fibers to one or more remote sites, typically other optical fibers. For example, where light is carried by one or more input fiber(s), the light may be transferred to, split between, or merged into one or more remote sites. Active or passive devices within the optical integrated circuit may also modulate or switch the input signal. Optical interconnects play an important role in fiber telecommunication, cable television links, and data communication. A waveguide is a type of optical interconnect.
For the most part, commercially available optical interconnects have been made of glass. Such interconnects, or couplers, are generally made by fusing glass optical fibers or by attaching glass fibers to a planar, glass integrated optical device that guides light from input fiber(s) to output fiber(s) attached to different ends of the device. Both approaches are labor intensive and costly. The cost increases proportionately with the number of fibers used due to the additional labor needed to fuse or attach each individual fiber. Such intensive labor inhibits mass production of these devices.
A further problem results from the mismatch in shape of the optical modes in the glass fiber and the integrated optical device. Glass fiber cores are typically round, whereas the channel guides tend to have rectilinear cross-sections. This mismatch tends to cause insertion losses when a fiber is butt coupled to an integrated optical device. Thus, there is a strong need for an integrated optical device or interconnect that can be easily attached to optical fibers with good mode matching.
As compared to glass structures, polymeric optical structures offer many potential advantages, and it would be desirable to have polymeric optical elements that could satisfy the demands of the telecommunications and data communications industries. Advantages of polymeric elements would include versatility of fabrication techniques (such as casting, solvent coating, and extrusion followed by direct photo-patterning), low fabrication temperatures (down to room temperature, allowing compatibility with a greater variety of other system components and substrates than is possible with the high processing temperatures characteristic of inorganic materials), and the potential ability to fabricate unique devices in three dimensions, all of which could lead to lower cost and high volume production.
Unlike glass optical interconnects, two-dimensional, polymeric channel waveguides are relatively easily produced. Numerous methods for fabricating polymeric waveguides have been developed. For example, electroplating nickel onto a master to form a channel waveguide mold and using photoresist techniques to form waveguide channels have been known for years. Cast-and-cure methods have supplemented older injection molding methods of forming polymeric channel waveguides. Following formation of the channel waveguide, further cladding and protective coatings typically is added inasmuch as polymeric waveguides generally must be protected from the environment to prevent moisture uptake or damage that could adversely affect performance.
The manufacture of other three dimensional, micro-optical elements has been quite challenging. Ion diffusion methods involve complex, multistep processes to build three dimensional structures. Photolithographic techniques, e.g., photoresist reflow, have been used to make lenses and the like. However, the range of shapes that can be made using lithography are limited by a number of factors including surface tension effects. Photolithography also is limited to the fabrication of elements whose optical axis is normal to the substrate upon which the element is fabricated. It is difficult, for instance, to make elements with accurate undercuts using photolithography.
U.S. Pat. No. 5,402,514 describes a different approach for manufacturing a polymeric, three dimensional interconnect by laminating dry films together. In these laminate structures, the outer layer(s) function as the cladding and the inner layers incorporate the optical circuitry. Single photon photopolymerization is used to photocure portions of each lamina. In order to build up three-dimensional circuitry using this approach, multiple exposure steps would be required to form each photocured lamina. Alignment of the layers during assembly to form the laminate structure could also prove problematic. The layers would also be subject to delamination if the bond quality between layers is poor.
Multiphoton polymerization techniques offer the potential to fabricate three dimensional optical structures more conveniently. Molecular two-photon absorption was predicted by Goppert-Mayer in 1931. Upon the invention of pulsed ruby lasers in 1960, experimental observation of two-photon absorption became a reality. Subsequently, two-photon excitation has found application in biology and optical data storage, as well as in other fields.
There are two key differences between two-photon induced photoprocesses and single-photon induced processes. Whereas single-photon absorption scales linearly with the intensity of the incident radiation, two-photon absorption scales quadratically. Higher-order absorptions scale with a related higher power of incident intensity. As a result, it is possible to perform multiphoton processes with three-dimensional spatial resolution. Also, because multiphoton processes involve the simultaneous absorption of two or more photons, the adsorbing chromophore is excited with a number of photons whose total energy equals the energy of an excited state of the chromophore, even though each photon individually has insufficient energy to excite the chromophore. Because the exciting light is not attenuated by single-photon absorption within a curable matrix or material, it is possible to selectively excite molecules at a greater depth within a material than would be possible via single-photon excitation by use of a beam that is focused to that depth in the material. These two phenomena also apply, for example, to excitation within tissue or other biological materials.
Major benefits have been foreshadowed by applying multiphoton absorption to the areas of photocuring and microfabrication. For example, in multiphoton lithography or stereolithography, the nonlinear scaling of multiphoton absorption with intensity has provided the ability to write features having a size that is less than the diffraction limit of the light utilized, as well as the ability to write features in three dimensions (which is also of interest for holography).
The use of multiphoton-induced photopolymerization has been described in Mukesh P. Joshi et al., “Three-dimensional optical circuitry using two-photo-assisted polymerization,” Applied Physics Letters, Volume 74, Number 2, Jan. 11, 1999, pp. 170–172; Cornelius Diamond et al., “Two-photon holography in 3-D photopolymer host-guest matrix,” OPTICS EXPRESS, Vol. 6, No. 3, Jan. 31, 2000, pp. 64–68; Cornelius Diamond, “OMOS: Optically Written Micro-Optical Systems in Photopolymer,” Ph.D. Thesis, January 2000; Brian H. Cumpston et al., “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” NATURE, Vol. 398, Mar. 4, 1999, pp. 51–54; T. J. Bunning et al., “Electrically Switchable Gratings Formed Using Ultrafast Holographic Two-Photon-Induced Photopolymerization,” Chem. Mater. 2000, 12, 2842–2844; Cornelius Diamond et al., “Two-photon holography in 3-D photopolymer host-guest matrix: errata,” OPTICS EXPRESS, Vol. 6, No. 4, Feb. 14, 2000, pp. 109–110; S. M. Kirkpatrick et al., “Holographic recording using two-photon-induced photopolymerization,” Appl. Phys. A 69, 461–464 (1999); Hong-Bo Sun et al., “Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin,” APPLIED PHYSICS LETTERS, Volume 74, Number 6, Feb. 8, 1999, pp. 786–788; Kevin D. Belfield et al., “Near-IR Two-Photon Photoinitiated Polymerization Using a Fluorone/Amine Initiating System,” J. Am. Chem. Soc. 2000, 122, 1217–1218.
The stability and quality of three dimensional optical structures made using multiphoton polymerization techniques remains a concern. Elements made to date have not been made in fully cured materials, thus, having poor stability, especially when exposed to light. Others are free standing and are not encapsulated, thus being sensitive to the surrounding environment and having potential stability issues relative to optical performance. It is also more challenging to achieve high circuit density when the boundaries between adjacent elements cannot be controlled with sufficient precision. Other methods have provided elements whose shape, index of refraction properties, and/or other or chemical physical properties degrade in a relatively short time.
Thus, there remains a strong need in the art for direct fabrication of three dimensional, stable polymeric optical elements with a high degree of precision, as desired. There is also a need for an approach that allows devices to be coupled together with low insertion losses and good mode matching.